Extended Type System

Extended Type System(ETS) is for consistent experience with some .NET types when working with PowerShell.

There's two approaches that PowerShell did for implementing the ETS.

• Intrinsic members for all objects, and intrinsic member for specific type of objects.
• Dynamic manipulation to members of an object.
• Potentially wrap an object as PSObject.

Intrinsic Members

There's two kinds of intrinsic members in PowerShell

• Object views: the mapping of a certain object.
• Property and methods: intrinsic properties and methods.

Object Views

Powershell distributes members to three parts for each object: extended, adapted and base.

• psobject: a MemberSet containing reflection source of members.

  • psobject.baseobject: a reference of the object itself.
  • psobject.properties: all properties from all sources(extended, adapted and base)
  • psobject.members: all members(including properties) from all sources(extended, adapted and base)
  • psobject.methods: all methods from type definition(such as .NET type), including raw methods of properties and indexers.

  $foo = 'I am foo'
  [object]::ReferenceEquals($foo, $foo.psobject.BaseObject) # True

• psbase: a MemberSet containing members of the original object.

  • the base members are defined ahead of time such as .NET types.

• psadapted: a MemberSet containing adapted members added by ETS.

  • members appended dedicatedly for a type by all means.

  $x = [System.Xml.XmlDocument]::new()
  $x.LoadXml("<foo><bar>123</bar></foo>")
  $x.psadapted.foo.bar # 123 #

• psextended: a MemberSet containing extended members added at runtime.

  • properties appended dynamically(conditionally) are extended property, such as PSPath for [FileSystemInfo] created from item cmdlets.
  • properties of a [pscustomobject] are extended properties.

• pstypenames: a CodeProperty equivalent to psobject.TypeNames. Returns a collection containing the type names of inheritance chain.

  $foo = 'I am foo'
  [object]::ReferenceEquals($foo.psobject.TypeNames, $foo.pstypenames) # True

Member Priority

1. psextended
2. psadapted
3. psbase

Intrinsic Methods

Intrinsic methods and properties are to mimic singular object and collection in a same form.

• Where: a method for filtering or slicing by condition. See [Where](../Object Manipulation/3.Where.md)
• ForEach: a method to perform iteration with certain logic or perform casting all items to target type. See [ForEach](../Object Manipulation/4.ForEach.md#Intrinsic%20ForEach)
• Count
• Length

Constructor

For all objects of System.Type, there's a intrinsic New method as constructor.

NOTE

Intrinsic members are not described as part of certain type definition, they're isolated from the object-oriented type system. Object views are visible to Get-Member -Force while intrinsic methods aren't.

A common example would be Process type.

(gps | select -First 1).GetType().Name # Process
(gps | select -First 1) -is [PSObject] # True
# -is checks underlying type too
(gps | select -First 1) -is [System.Diagnostics.Process] # True

The functionality of ETS achieved benefits:

• Allow custom behaviors like formatting, sorting for your custom type or object.
• Dynamic interpreting a HashTable to any PSObject
• Accessibility to the underlying object.
• Manipulation on extended members.

ETS Members

To represent meta of ETS members, PowerShell has an abstract class called PSMemberInfo, each concrete member info type is derived from it.

public abstract class PSMemberInfo { }

The facts of ETS members:

• each member has a type inherited from PSMemberInfo.